Paper making process requires the rapid and accurate measurement and control of consistency of the stock slurry during several stages in the process to ensure a uniform high-quality final product. Consistency, usually expressed in percentage, is defined as the weight in grams of oven-dry material in 100 grams of a pulp-water mixture. Prior art measurement devices have suffered from the problems of stock species and stock flow velocity, since different stock species and flow velocity produce errors in measuring stock consistency.
Different species of trees or other fiber sources exhibit fibers of differing lengths and diameters which are characteristic of their particular origin. The handling of the fiber during the paper making process further dimensionally alters the fiber from the degree of refining, type of process being used, mechanical, chemical, etc.
Variations in fiber length and cross-section invariably cause an error in measurement--the direction and magnitude of the error being dependent on the measurement principle being used.
Instruments of a mechanical design utilizing shear as their principle of measurement typically exhibit a decrease in reading due to the apparent reduction in friction when measuring stock with short fiber length and small diameter, as would be found in the hardwood species. Conversely, softwood varieties with longer, larger fibers increase the apparent friction, thus increasing the reading for any given consistency.
Optical instruments employing backscatter as their principle of measurement generally exhibit an error in the reading in the opposite direction to that of the mechanical devices. Hardwood fiber, having a larger surface area for a given consistency than softwood fiber, causes the optical instrument's output to read higher due to the backscatter characteristics generally associated with an increase in consistency.
Velocity of the stock slurry also causes measurement error when one is attempting to gauge consistency. Usually, the measurement device cannot detect a difference between a changing flow rate or consistency, requiring that some compensation be made for the effects of flow velocity.
Therefore, there is a need for a consistency measurement device of the backscatter principle having the ability to compensate for variations in fiber size and stock flow velocity.